1. Field of the Invention
The present invention relates to an impregnated soft, flat gasket, particularly for use as a cylinder head gasket for internal-combustion engines, to a manufacturing method for producing such gaskets, and to an article useful in the manufacturing method.
2. Discussion of the Prior Art
Gaskets for internal-combustion engines, particularly cylinder head gaskets, but also exhaust flange gaskets or other flat seals are made of non-woven fiber mats, which may be metal reinforced, and are impregnated primarily to increase their strength, improve their resistance to the media to be sealed and improve their sealing behavior. Impregnating agents are, for example, those according to German Auslegeschrift No. 2,304,505, which corresponds to U.S. Pat. No. 3,970,322 to Stecher et al, which are cross-linkable and solvent-free impregnating agents, and are usually thermally cross-linked in the gasket, such as by means of peroxides contained therein. Preferred impregnating agents are recited to be liquid polybutadienes, polyacrylates, unsaturated polyesters or liquid silicone elastomers which undergo cross-linking by polyaddition via carbon double bonds without the release of gaseous substances. In practice, it is also known, for example, from U.S. Pat. No. 4,291,087 to Warburton, Jr., to cross-link the impregnating agents, which are composed of polyacrylates, by the influence of high-energy electron beams.
When installed between the sealing faces of an engine block and a cylinder head, cylinder head gaskets are stressed to different degrees depending on a predetermined compressive pressure which is a function of engine design. To compensate for this, it is customary, for example, according to the earlier-mentioned German Auslegeschrift No. 2,304,505, to keep the regions of the soft material below the metallic flanges free of impregnating agent, particularly at the combustion chamber openings, to thereby increase the sealing pressure in these stressed zones.
German Auslegeschrift No. 1,074,341 teaches to locally vary the deformation characteristic of a gasket in adaptation to its particular use so as to equalize the sealing pressure and avoid cylinder warping. According to a preferred solution disclosed in this reference, a soft gasket material is impregnated to influence its ultimate deformability by varying the level of absorption of an impregnating agent from location to location.
It has been found, however, that it is difficult in practice to impregnate soft-material gasket mats by partial absorption or by varying the level of absorption from location to location. Thus, according to the German Auslegeschrift No. 2,304,505, cut-out gaskets are provided with edge casings and are preferably impregnated by immersion, the casings shielding the gasket material therebeneath. In spite of the shielding casings, however, penetration of impregnating agent under the casings cannot be completely avoided by this process. Likewise, if the gaskets are partially impregnated as disclosed in the German Auslegeschrift No. 1,074,341, the liquid impregnating agent has been found to gradually spread over the entire gasket and result in the gasket being undesirably impregnated over its entire surface area.